Driving a Mechanical Speedometer with a Raspberry Pi Part 1 of 2: The Mechanical Bits

February 20, 2019

Mechanical things are so satisfying. We love taking an old piece of equipment and making it work with products we sell.

Back in the day, speedometers were driven by spinning cables. These cables would typically run from the transmission via a flexible steel cable in a sheath to the speedometer. Ever been in an older car where the speedometer bounces? That is usually because the speedometer cable is about to fail.

We have been looking for interesting ways to represent data, and we were thinking it would be fun to use an old odometer to show how many orders we have received. To do this, we would need an old speedometer/odometer assembly, a motor, a way to drive the motor, and a way to couple the motor to the speedometer.

We found an old speedometer on Kijiji for $10 and verified that the odometer worked. The odometer came with 1/10 of a mile on it! We believe it came off an exercise bike...which must have collected a lot of dust! On the back of the speedometer, there is an input shaft with a square hole in it. To drive the speedometer, you just spin the input shaft. To test a speedometer, you can use a drill.

Now we had to find a way to drive the speedometer. It turns out that our Beefy Hobby Motors work really well for this application. At first we just took the input shaft and wrapped some tape around it. This sort of worked, but it was clear we would need something better, as there was a lot of slippage. We needed this thing to be precise!

Fortunately, we were able to find a speedometer cable repair kit on Amazon! With this, we got the cable with a square end on it. We were able to cut the cable with heavy duty wire cutters and put one end of it into a shaft coupler. The coupler attached to our Beefy Hobby Motor, and the other end of the cable went into the speedometer.

Everything worked great, but we needed a project enclosure. Since we have a thing for laser cut boxes, we made a 78x78x70mm box out of 3mm birch plywood and added cutouts for the speedometer, motor, and a mounting point for a Raspberry Pi. As always, we used MakerCase to lay out the case, and then added in the required holes via Inkscape.

Here are some pictures showing the box layout and the parts. In the layout, the piece floating in the lower left is the removable top.